The invention relates to a new electrode comprising a compound of perovskite structure. An electrode of this type may be used particularly in cells for electrolysis of alkali-metal halides or alkali-metal chlorates and perchlorates.
The use of a metal of the platinum series electrode employed in electrolysis has been known for a long time. Platinum and platinum alloys are known to have considerable advantages due to their long life, particularly in electrolysis of alkali-metal halide solutions, especially sodium chloride solutions. However, such anodes made of solid metals from the platinum series are prohibitive in price, with the result that graphite anodes have generally been used until quite recently. Unfortunately, graphite anodes wear away rapidly in use. On the one hand, this means that they have to be replaced frequently, and on the other hand, it means that conditions encountered in electrolysis vary to the extent that wear on the anodes changes the interpolar distance. This defect is all the more serious in that, for economic reasons and particularly reasons of industrial production, it is desirable to increase current densities and reduce interpolar distances.
For this reason, attempts have been made to make anodes with dimensions which do not vary substantially during operation. The first arrangement used for this purpose was a conductive substrate which was chemically inert in the electrolytic medium and covered with an active layer. The material used for the substrate was generally a valve metal capable of forming a blocking layer of oxide in electrolytic solutions, and to protect its surface from corrosion in places where the metal of the active layer is porous. In particular, an anode was used, made of a substrate of titanium covered with an active layer consisting of a metal from the platinum series. Unfortunately, the titanium oxide which forms to protect the titanium substrate chemically is itself non-conductive. Thus, French Pat. No. 2,088,659 proposed making the blocking layer of a precious metal on a carrier which was a good conductor of electricity. However, such a solution has the disadvantage of being economically expensive. Attempts have also been made to replace the active layer of noble metal or compounds of noble metal with other products, e.g., oxides of noble metals.
It has also been proposed, e.g., in French Pat. No. 2,128,667, that an anode comprising a substrate, e.g., of titanium should be covered with a thin film of an electrically conductive coating such as a metal of the platinum group or an oxide or sulphide of such a metal, with an external layer of a compound of perovskite structure applied to it. Unfortunately, such perovskite structures are generally soluble in an acid medium and non-conductive. In electrolysis of sodium chloride, slight oxidation of the water at the surface of the electrodes leads to the formation of a diffusion layer which is even more acid than the medium, which makes the above disadvantage even more serious. It has, indeed, been proposed to increase the activity of such a structure by doping it, e.g., with strontium or nickel in the case of lanthanum cobaltite. Unfortunately, the solubility of the compound of perovskite structure is further increased.
Moreover, in the cases where a valve metal such as titanium is used as the substrate, there is reason to prevent the substrate from being passivated by the growth of a layer of non-conductive titanium dioxide. Thus, in Bouy et al. U.S. application, Ser. No. 486,052, filed July 5, 1974, commonly owned, there is disclosed a method for the deposition of a layer of perovskite compound directly on the substrate, using a solution of a cobalt salt which gives a cobalt oxide by thermolysis. The appropriate compound of perovskite structure is of the general formula A.sup.a B.sup.b O.sub.3 wherein A represents a metal ion and B is another metal ion, A and B being linked by the equation a + b = 6, wherein a and b respectively represent the classical valencies or ionic charges of ions A and B. An account of the structure of this type of compound is given, e.g., in "Crystal Structure" by Wyckoff, Vol. 2, 2nd edition (1964), Wiley and Sons, pages 390 to 402.
It has been noted in particular that compounds of the formula Ln CoO.sub.3, in which Ln is a metal of the lanthanum family, behave well in slightly acid environment which enables them to be used as anodes in certain electrolytic media. But it has been observed that corrosion in an acid medium decreases, the more acid the character of the rare earth metal used.
This led Bouy and Cheradame to propose a cobaltite of rare earths in their U.S. Pat. No. 3,917,525, granted Nov. 4, 1975. The cobaltite comprised at least two rare earths, one of the rare earths having a high atomic number and not giving a compound of solely perovskite structure when combined with cobalt. But although all these compounds give interesting results, the durability obtained is still somewhat less than desired considering the requirements for modern electrolytic cells. Moreover, it has been noted that if the thickness of the layer is increased to obtain greater durability, the tension of the electrode is increased simultaneously, thus making this solution difficult to apply industrially.
It is, accordingly, an object of the present invention to provide electrodes for electrolytic cells which do not have the shortcomings of prior art electrodes.
It is also an object of the present invention to provide an electrode for an electrolytic cell which employs a cobaltite of perovskite structure, which electrode possesses excellent durability.
It is a further object of the present invention to provide electrodes for electrolytic cells, which electrodes have excellent resistance to corrosion.
Further objects will be apparent to those skilled in the art from the present description.